1. Field of the Invention
The present invention relates generally to metal-ceramic composite substrates, and more specifically to brazing materials that can effectively be used in joining metals to ceramic substrates.
2. Description of the Prior Art
Several methods are known in the art of joining metal such as copper plates to ceramic substrates such as aluminum nitride (AlN) and two typical examples are the active metal brazing process described in Japanese Patent Laid-Open No.166165/1985 entitled xe2x80x9cA method of Joining Nitride-Base Ceramics to Metalsxe2x80x9d and the process in which a copper plate is directly joined to a modified surface of an aluminum nitride substrate (as typically described in Japanese Patent Laid-Open No. 163093/1981).
The active metal brazing process provides a higher strength of joint than the direct joining process and the resulting joint has the desired charactristics such as high durability to repeated heat cycles. Hence, the active metal brazing process currently finds extensive use in the joining of copper plates to non-oxide-base ceramic substrates, say, nitride ceramic substrates.
Two brazing materials are commercially used in the practice of the active metal brazing process; one of them is composed of Ag, Cu and an active metal selected from among Ti, Zr and Hf (as described in Japanese Patent Laid-Open No.166165/1985) and the other is an active metal paste material composed of Ag, Cu and titanium hydride (as described in Japanese patent Laid-Open No.101153/1991).
Circuit substrates produced by joining copper plates to both sides of an aliminum nitride substrate with the aid of these brazing materials have already been commercialized.
In many brazing materials, an active metal is added to a structure comprising 72% of Ag and 28% of Cu, and accordingly the joining process is carried out at a temperature higher than the eutectic point of 780xc2x0 C., preferably, at a temperature of around 850xc2x0 C.
However, most recent circuit substrates are required to perform on larger electrical power and to meet this requirement, it has been desired to develop circuit substrates that are not only provided with good heat dissipating and electrical insulating properties but which also exhibit higher strength and thermal impact resistance.
An object, therefore, of the invention is to provide a brazing material that is suitable for use in the manufacture of power module composite substrates having no non-contact portions and which is capable of improving their charactristics than the conventional brazing materials.
The present inventors conducted intensive studies in order to attain the stated objects and found that joining processes could be carried out at a temperature of 780xc2x0 C.xcx9c870xc2x0 C. lower than the eutectic point of 875xc2x0 C. by using active metal brazing materials added with 90% of Ag and 9.5% of Cu or 0.5% of an active metal (TiO), and that the jointed composite substrates had no non-contact portions.
The present invention has been accomplished on the basis of this finding.
According to a first aspect of the invention, there is provided a metal-ceramic composite substrate produced by joining a metal plate to a ceramic substrate by using a brazing material of which the solid content comprises 90.0xcx9c99.5% of Ag, 0xcx9c9.5% of Cu, 0.5xcx9c4.0% of an active metal on a weight basis.
Said ceramic substrate is at least one kind of ceramic substrate selected from a group consisting of Al2O3, AlN and Si3N4.
Said active metal is at least one active metal selected from a group consisting of Ti, Zr and Hf.
There is provided a process for producing a metal-ceramic composite substrate by joining a metal plate to a ceramic substrate,which comprises the steps of:
adding 10-14 parts by weight of a vehicle to 100 parts by weight of a powder of which the solid content comprises 90.0xcx9c99.5% of an Ag powder, 0xcx9c9.5% of a Cu powder and 0.5xcx9c4.0% of an active metal powder on a weight basis;
blending the respective ingredients to form a brazing material in a paste form;
applying the brazing material to the ceramic substrate;
superposing a metal plate on the applied brazing material and heating the assembly at a temperature not higher than the melting point of the brazing material to form a joint of the metal plate and the ceramic substrate; and
applying an etching resist on the metal plate of the joint to form a circuit pattern and etching the metal plate to form a metalized circuit.
Said heating temperature of the assembly is in the range of 780xc2x0 C. to 870xc2x0 C. which is not higher than the temperature of the melting point of the brazing material.
According to another aspect of the invention, there is provided a brazing material for use in joining a metal plate to a ceramic substrate, which is in a paste form prepared by blending 10-14 parts by weight of a vehicle with 100 parts by weight of a powder of which the solid content comprises 90.0xcx9c99.5% of an Ag powder, 0xcx9c9.5% of a Cu powder and 0.5xcx9c4.0% of an active metal powder on a weight basis.
There is provided a brazing material for use in joining a metal plate to a ceramic substrate, which is in a paste form prepared by blending 10-14 parts by weight of a vehicle with 100 parts by weight of a powder of which solid content comprises 90.0xcx9c99.5% of an Ag powder, 0xcx9c9.5% of a Cu powder, 0.5xcx9c4.0% of an active metal powder and 0.0xcx9c0.9% of a titanium oxide powder on a weight basis.
The brazing material to be used in the invention has an Ag content of 90.0xcx9c99.5 wt %. If the Ag content is less than 90 wt % unsuitable composite substrates having non-contact portions will be produced.
The Cu content of the brazing material is less than 9.5 wt %. The reason why the Cu content is smaller than that of the conventional Agxe2x80x94Cu brazing material is that when the assembly is heated the Ag component and the contact surface of the copper plate are reacted to create the eutectic crystalization.
The active metal that can be used in the invention is at least one element of group IVa of the periodic table as exemplified by Ti, Zr or Hf.
These active metals may be added either in an elemental form or as a hydride and the preferred amount of their addition ranges from 0.5 to 4.0 wt %.
Below 0.5 wt %, a nitride layer will form in an insufficient amount to provide the necessary adhesive strength; beyond 4.0 wt %, the adhesive strength is increased, but, on the other hand, cracking is prone to occur after joining a metal plate to a cearmic substrate.
Titanium oxide is added as TiO or TiO2 in an amount ranging from 0.0 to 0.9 wt %. The titanium oxide may be amorphous or crystalline.
The present inventors confirmed by experimets that adding these amount of titanium oxide to a brazing material of the composition described above contributed to improvements in various characteristics of the resulting composite substrates such as resistance to repeated heat cycles, flexural strength, deflection and resistance to the passage through a furnace. A probable reason for these contributions would be that the TiO2 or TiO added to the brazing material is dispersed uniformly to reduce the concentration of stresses.
An alloy having the above-defined composition may be directly used as the brazing material of the invention. Alternatively, a powder comprising the particles of the respective ingredients may be mixed with an organic solvent to form a paste of the brazing material. If a metal member is to be simply joined to a ceramic member, the brazing alloy material in the form of a plate or a foil may be used. If an electronic circuit is to be formed on a ceramic substrate, a paste of the brazing material is preferably used.
In order to prepare the paste of brazing material, 55-75 parts by volume of an organic solvent such as terpineol, toluene, methyl cellosolve or ethyl cellosolve may be mixed with 25-45 parts by volume of an organic binder such as PMMA, methyl cellulose or ethyl cellulose to form a vehicle, which is then added in an amount of 10-14 parts by weight to 100 parts by weight of the powder comprising the particles of the respective ingredients for the brazing material.
If less than 10 parts by weight of the vehicle is used, the resulting paste is so viscous that blurred prints will tend to occur. If more than 14 parts by weight of the vehicle is used, the resulting paste is so low in viscosity that print washout will readily occur.
The present invention is characterized in that the assembly is heated in the furnace at a temperature lower than the eutectic melting point of the brazing material, preferably at 780xc2x0 C.xcx9c870 xc2x0 C. according to the composition of the brazing material for the reasons mentioned below.
In case that the conventional brazing material formed of 72% of Ag and 28% of Cu added with an active metal Ti is used, the joining is intiated at a temperature higher than the eutectic melting point of Agxe2x80x94Cu, that is, of 780xc2x0 C. Accordingly, in practice, the joining is carried out at a temperature of around 850xc2x0 C. In the present invention, the amount of Ag is more than 90 wt %. If Ag is 90 wt %, Cu is 9.5 wt % and Ti is 0.5 wt %, the eutectic melting point should be a temperature more than 875xc2x0 C. in consideration of the phase diagram of silver-copper, so that the joining have to be carried out at a temperature of around 900xc2x0 C. in order to obtain a perfect joint in the conventional process.
If the brazing material of the present invention only is heated at a temperature lower than said eutectic melting point (875xc2x0 C.), it may not be molten but merely sintered. However, in case of using a copper plate as a substrate for the power module as in the present invention, said brazing material and the copper plate are brought into contact with each other, so that when the assembly is heated Ag in the brazing material and Cu in the copper plate are reacted eutecticlly gradually from said contact portion in the micron order, Ag is diffused gradually into the copper plate started from said reaction and the eutectic structure is formed on the whole contact surface.